Mark v saturation mod

What is it and what does it do.

Probably easier to go thru the thread for details. But its a tube swap that mellows out Ch 3

V saturation mod thread

Here is a brief response. General synopsis: The Mark V90W is one of the few amps that can be tuned with change of preamp tubes. This amp responds quite well to short, medium or long plate 12AX7 tubes in various positions. The one trick to allow for more gain saturation and to cut out the ice pick tendency is to use a tube with less gain characteristics in the upper frequency range. As it is the basic triode tube is a non-linear device that will have a different gain effect based on frequency.

The saturation mod can consist of one or two things.

The original post was for change in V6.

• Replace the V6 tube with a NOS JAN/Phillips 12AT7 (original concept). Similar desire was to allow for more tube saturation effect.

Due to experimentation, it was discovered that V4 was an alternate.

• 2. Replace the V4 tube with a NOS JAN/Phillips 12AT7. This aids to change the gain structure of the higher frequencies. The low end characteristic will remain the same. Helps to reduce higher order harmonics in the higher frequency range that cuts on ice pick characteristics of some Mark V amps. Also enhances the IIC+ characteristic. This improves the gain structure so you can use more treble and gain settings without going into ice pick tones.

I would not do both, just pick one or none.

It has been a few years since I contributed to this thread. I did go overboard with hard mods but those are not necessary. I had more of an ice pick issue with CH3 and learned a great deal with some minor changes. Some of which I restored but a few I left as is. Keep it simple and only try the tube swap change.

Thanks for the response. I have a couple of JAN Phillips 12AT7 NOS Tubes with the blue printing on the tubes dated to early 90's are these the right ones?

Yes, that is correct.

JAN-12AT7WC is the part number. Phillips ECG will also appear below this.

bandit2013 said:

Here is a brief response. General synopsis: The Mark V90W is one of the few amps that can be tuned with change of preamp tubes. This amp responds quite well to short, medium or long plate 12AX7 tubes in various positions. The one trick to allow for more gain saturation and to cut out the ice pick tendency is to use a tube with less gain characteristics in the upper frequency range. As it is the basic triode tube is a non-linear device that will have a different gain effect based on frequency.

The saturation mod can consist of one or two things.

The original post was for change in V6.

• Replace the V6 tube with a NOS JAN/Phillips 12AT7 (original concept). Similar desire was to allow for more tube saturation effect.

Due to experimentation, it was discovered that V4 was an alternate.

• 2. Replace the V4 tube with a NOS JAN/Phillips 12AT7. This aids to change the gain structure of the higher frequencies. The low end characteristic will remain the same. Helps to reduce higher order harmonics in the higher frequency range that cuts on ice pick characteristics of some Mark V amps. Also enhances the IIC+ characteristic. This improves the gain structure so you can use more treble and gain settings without going into ice pick tones.

I would not do both, just pick one or none.

It has been a few years since I contributed to this thread. I did go overboard with hard mods but those are not necessary. I had more of an ice pick issue with CH3 and learned a great deal with some minor changes. Some of which I restored but a few I left as is. Keep it simple and only try the tube swap change.

Click to expand...

Does this apply to all years of the Mark V 90W or just the early years?

Also, which of the 2 tube swaps did you find most useful?

Thanks,

Roger

rtcook said:

Does this apply to all years of the Mark V 90W or just the early years?

Also, which of the 2 tube swaps did you find most useful?

Thanks,

Roger

Click to expand...

You can apply this to any model year of the Mark V90. The V4 has the most effect. I tried the V6 but thought it reduced what I liked about the gain structure. For reference, mine is a 2012 and should be much the same as the current production models. The first release in 2009 had a different tone stack and some added parts not on the schematics or PCB. 2010 is the year they made the changes on the PCB. If there were other mods after that time period, it is not advertised.

For some reason or another, I could not set the treble past 9am or the gain too far as it would result in ice pick tones that were not good to listen too. I used to think it was the guitar or the speakers but turns out when I slaved the Mark V90 into the Roadster, it was ice pick heaven that way as well. If you have a brittle sounding Mark V on the lead channel, the 12AT7 in V4 greatly helps to cut that issue so you can make use of the treble and gain controls without the ice pick.

What it does in V4 position: it reduces the higher frequencies above the treble range. In other words, it is cutting back on higher order harmonics. This allows you to push the gain setting much higher without getting into the ice pick sound. Go ahead and try your amp with the gain control maxed. It that sounds good to you, you probably do not need to use the 12AT7 in V4. If it drills into your head without any mercy and sounds like breaking glass in your ears, the 12AT7 will remove that effect and add in some compression and gain saturation you cannot get with the 12AX7 preamp tube. Since the lead circuit of the Mark amp is more asymmetrical, it can be unforgiving in the treble and higher frequenices. Works on all voices, IIC+, IV and Extreme.

In the V6 position, I did not get the same effect. More like a gain reduction since the last gain stage is more for voicing the tone than for the actual drive or distortion of the signal.

The actual gain structure of the lead channel is created by the V5a to V4b. the V5a has a mild gain characteristic but its load line is set to clip early. It follows into V4b that has a very high gain characteristic. Since the triode is non-linear in nature, the harmonics of the fundamental frequencies will be multiplied. The harmonic I am referring too is the multiplied order. If I consider the even harmonics, Say 1kHz generates 2kHz, 4kHz, 8kHz, 16kHz, 32kHz and so on. Due to the higher gain factor of that particular stage, roll off of the upper harmonics will extend past the 8th harmonic if not higher. The ice pick characteristic of high order harmonics is not necessarily the function of the main drive circuit. This exact triode pair has been used since the IIC model but with some minor tweaks here and there. There is one difference the Mark V has compared to the earlier models, addition of an extra gain stage V3A and then V6A. There are a few voltage divider circuits between V3A and V6A most of it is for the reverb circuit as that is where the reverb feed and return are tapped from for the lead channel. Had Mesa stuck with the traditional arrangement (GEQ after the FX loop) it probably would not be much of an issue.

The issue is mostly related to the GEQ circuits that follow. The front end of the GEQ and the back end of the GEQ are all transistors. Even if you turn off the GEQ, the signal still passes through the GEQ circuits. Turn of the FX loop into bypass, you still have the transistors in the signal path. So if there are high order harmonics in the signal, transistors when driven will generate the odd order harmonics that tend to sound very harsh and unpleasant. V4 is the prime candidate that has the highest gain factor in the entire amp. More prone to generate high order harmonic content that can lead to shot-noise effect but that is more related to the transistors used in the GEQ than the tube circuit. Since the signal level for the FX send is also derived by the same transistors (back end of the GEQ), there tends to be an impedance mismatch to the effect in the loop. This results in tone suck or buffers being overdriven into clipping. Sure the signal level may matter but the impedance issue is worse.

If it is for trying to correct the FX loop characteristics, the only way is to use pedals or effects that are tolerant to line level (usually +8dBu will be specified in the effects manual). The other aspect that I did not know is the Strymon effects seem to auto correct for impedance mismatch. That is how I perceive it.

If either of these positions are not to your liking, you can use the 12AT7 in the phase inverter position V7. That also has an interesting effect. That trick comes from the users of the MKIII and IIC people. I tried that with the JP2C too worked good with that.

I like the 12AT7 in the V4 position. Just takes a little of the "graininess" out of channel 3. Note: it reduces gain also, so you have to dial up gain a bit to compensate for slight saturation loss.

Thank you greatly for this detailed information. I will try your suggestions.

Roger

I don’t have any nos tubes, would a 5751 work?

Also I have the 25 watt version, would it work for this and which tube would this be?

Allstarrme said:

I don’t have any nos tubes, would a 5751 work?

Also I have the 25 watt version, would it work for this and which tube would this be?

Click to expand...

I have tried a NOS JAN/GE 5751 in the V4 position of the Mark V90. Interesting response but the 12AT7 just seemed better. That being said, the smaller platforms to not have the same design in how the cascaded gain stages are arranged. With the 90W version, it is the 3rd gain stage that has the gain control and the 4th gain stage as the high gain overdrive or saturation position. The V90 has V5A as the 3rd gain stage and V4B as the 4th gain stage. The other triodes with those tubes are for the reverb send/return. An alternate tube in V4 will affect the gain structure of the lead channel and have a slight effect on the reverb send. The other channels are not effected in relation to the gain characteristics.

This is not possible to achieve with the Mark V:25 or V:35 since the 3rd and 4th gain stages are shared with the other channel. That would be V2. Note that CH1 also uses V2A for the 2nd gain stage for crunch. Making a change in V2 with a different tube will affect CH1 crunch characteristics. Not sure what the final effect would be, better or worse. Some have tried rolling in the 12AT7 in the V:25 and V:35. You will have to dig into the thread to find out their responses. I do not own the V:25 or V:35 to comment.

I have the V25 and never felt the need to try any tube rolling, the high gain channel sounds monstrous without being harsh. Not sure what the difference is between hi gain channels of V90 and V25 but there is a difference. The V90 is less smooth than the V25, at least to my ears.

I’m still waiting for a shootout video which compares Clean, Bold, Crunch, IIc+, IV between the new VII and V90! Really curious how those two compare overall